text: The term  is known as the surface term. This term, also based on the strong force, is a correction to the volume term.
  The volume term suggests that each nucleon interacts with a constant number of nucleons, independent of A. While this is very nearly true for nucleons deep within the nucleus, those nucleons on the surface of the nucleus have fewer nearest neighbors, justifying this correction. This can also be thought of as a surface-tension term, and indeed a similar mechanism creates surface tension in liquids.
  If the volume of the nucleus is proportional to A, then the radius should be proportional to  and the surface area to . This explains why the surface term is proportional to . It can also be deduced that  should have a similar order of magnitude to .